This invention relates to a method of preparing silicon esters. For purposes of the invention, the term silicon ester is intended to include acyloxy silanes and acyloxy siloxanes. More particularly, the invention is directed to a process in which an allyl ester is contacted with a silicon hydride, and in the presence of a Group VIII metal catalyst.
Acetoxy silanes and acyloxy silanes are useful materials as silylating ingredients, and as cross-linkers in the production of moisture cure silicone sealants. Known methods of preparing acetoxy and acyloxy silanes involve the reaction of chlorosilanes with acetic acid, acetic anhydride, organic carboxylic acids, organic anhydrides, and carboxylate salts. Other known methods require the reaction between carboxylic acids and silazanes; exchange reactions involving alkoxysilanes and organic anhydrides; and reactions of carboxylic acids and aminosilanes. Such processes, however, often result in the formation of complex mixtures which cannot be readily separated. Acyloxy silanes have been produced with carboxylic acids, hydridosilanes, and a transition metal catalyst, however, the generation of hydrogen gas has limited the use of this process commercially. In U.S. Pat. No. 4,746,750, issued May 24, 1988, a process is described for preparing silyl ketene acetals from allyl 2-organoacrylates and trisubstituted silanes. The process of the present invention is similar to the process described in the '750 patent. The basic difference, however, between the method of the present invention and the method of the '750 patent, is that the '750 patent requires a molar ratio between the 2-organoacrylate and the silane of greater than 1:1, typically 1:2. In accordance with the method of the present invention, however, there is required a ratio less than about 1:1. As a result, it is only possible in accordance with the method of the present invention, to prepare and isolate a silicon ester, and not a silyl ketene acetal as in the '750 patent.